At the present time, there are a wide variety of known mass spectrometers, including quadrupole, magnetic deflection, TOF (time of flight), Fourier transform and other types of mass analyzers.
Various techniques have been developed for ionizing substances of interest in a liquid solution and introducing the ions into a mass analyzer. Older techniques include fast atom bombardment, ion evaporation and thermospray. More recently, the so-called electrospray ionization (ESI) technique has been developed. In this process, liquid is directed through a capillary tube, the end of which is connected to one pole of a high voltage source. The end of the capillary plate is spaced from an orifice plate through which ions travel into the mass analyzer vacuum chamber. The orifice plate is connected to the other pole of the high voltage source, so as to generate an electric field. This causes charged droplets to travel towards the analyzer. Solvent carried by the droplets evaporates, to leave ions of the substances of interest. Typically an ESI source is operated at solvent flow rates around 1-10 .mu.L/min which requires the use of a pump. Another known ion source is an Atmospheric Pressure Chemical Ionization (APCI) source. For an APCI source, solvent containing analyte is sprayed into a heated tube where it is vaporized. Solvent and analyte are then ionized by a corona discharge at atmospheric pressure.
Older techniques, or even the more modem ESI and APCI techniques, have generally been applied to liquids or samples which are treated as stable, i.e. they are assumed to have a substantially constant composition, which will remain unchanged for at least as long as it takes to process the sample through the sample introduction system and ion source. In other words, no attempt has been made to use this technique to analyze samples where reactions may be occurring in a relatively short time frame, e.g. of the order of seconds, fractions of a second or several milliseconds. Conventional mass spectrometric techniques, including the ESI technique, simply do not permit reactions occurring on such a short time scale to be captured, since they essentially require the sample to be stable, at least while it is held in some supply vessel, before passing through the capillary to the electrospray nozzle.
Attempts have been made to use mass analyzers to capture changes in concentrations of reactants, as reactions or other changes take place. This can be done by observing the decrease of reactant ion intensity or the appearance of product ions in the mass spectra. In some cases reaction intermediates might be observed. However, conventional techniques, outlined below, are relatively crude, and can only measure events taking place on fairly large time scales, for example of the order of minutes or longer.
Thus, one known technique was described by R. Chavez et al. in a presentation at the 44th ASMS Conference on Mass Spectrometry and Allied Topics, Portland, Oreg., in May 1996. This described manual mixing of reactions with aliquots taken at various times. As will be appreciated, this necessarily resulted in time scales of the order of minutes.
A further, manual mixing technique, with mixing carried out in a vessel connected to an ESI MS, was described by E. D. Lee et al. in the J. Am. Chem. Soc. 116 5250-5256, 1994. Again, the time scale was relatively large and was of the order of 0.1 to 10 minutes.
A fundamental difficulty with all of these techniques is that they assume that the electrospray apparatus will have a single capillary to which some sort of vessel must be connected to supply the liquid. This necessarily means that the vessel has some sort of distinct holding time, i.e. time in which the liquid takes to pass through the vessel, which limits the minimum time periods for measuring reactions.
Recently, there has been a proposal for providing on-line mixing coupled to an electrospray apparatus. This is described by J. W. Sam et al., J. Am. Chem. Soc. 116, 5250-5256, 1994, and J. W. Sam et al., J. Am. Chem. Soc. 117, 1012-1018, 1995. In these proposals, the ability to vary the reaction time by adjusting the flow rate is identified. The apparatus relies on a conventional mixing technique, and there is no discussion as to the effect of the length of the electrospray capillary on reaction times. As such, this provides a so-called mixing chamber, with a relatively large volume of 5 .mu.L. Consequently, the apparatus only enables a relatively large time scale of the order of a few seconds to a few minutes to be used.
Moreover, the earlier techniques relying on manual mixing are difficult to carry out and also are slow. Often, they will require considerable manual handling and dexterity. Such techniques would require the person carrying out the test to take samples in a reliable manner and at exactly the right time periods. It is difficult to ensure that this is done consistently and accurately.